Fluid Catalytic Cracking (FCC) is one of the key processes employed in petroleum refineries for converting heavy vacuum gas oil into lighter products namely gasoline, diesel and liquefied petroleum gas (LPG). Processing of heavy residues e.g. atmospheric and vacuum bottoms are increasingly being practiced in the FCC Unit for enhanced conversion of residue. Heavy residues contain higher amount of Conradson carbon residue (CCR), poisonous metals e.g. sodium, nickel, vanadium and basic nitrogen compounds etc., all of which have significant impact on the performance of FCC unit and the stability of its catalyst.
The high CCR of the feed tends to form coke on the catalyst surface, which in turn brings down the catalyst activity and its selectivity. Moreover, the higher deposit of coke on the catalyst increases the regenerator temperature and therefore catalyst/oil ratio reduces to maintain the heat balance of FCC unit. The FCC catalyst can tolerate a maximum temperature of up to 750° C., which limits the CCR of feed that can be processed in FCC unit. At present, FCC unit with two stages regenerators and catalyst coolers can handle feed CCR up to 8-wt % economically.
Nickel, vanadium and sodium are also present in large quantity in the residual feed. The poisoning effects of these constituents are well known in the FCC art. In the past, there have been some efforts to passivate the damaging effects of nickel and vanadium on the catalyst. These efforts have resulted only with some success in the passivation of nickel. Thus, by the known methods, it is presently possible to handle up to 30 PPM of nickel on the feed and up to 10,000-PPM nickel on the equilibrium catalyst. Similarly, with the known processes, vanadium up to only 30 PPM on feed and 15000 PPM on the equilibrium catalyst can be handled economically. These above limits pose serious problem of residue processing capability of FCC unit. As such, huge quantity of metal laden equilibrium catalyst is withdrawn from resid FCC(RFCC) unit to keep the circulating catalyst metal level within the tolerable limit. As regards the passivation of basic nitrogen compounds, suitable passivation technology is yet to be found.
In addition to the developments of passivation technologies, there have been some important design changes made in FCC for efficient residue processing. One such design change is the two-stage regeneration in place of single stage regeneration. U.S. Pat. No. 4,064,038 teaches the advantages of two-stage regenerator and its flexibility to handle additional feed CCR without requiring catalyst cooler. However, even with two-stage regenerator of U.S. Pat. No. 4,064,038, there is limitation to increase feed CCR above 4.5-wt % and vanadium above 15-20 PPM on feed.
It has been suggested in the art to use a separable mixture of catalyst and demetallizing additive particles. For example, in U.S. Pat. Nos. 4,895,637, 5,021,222 and 5,110,775, suggest a physically separable mixture of FCC catalyst and demetallizing additive having sufficient differences in their settling velocities so as to cover a segregation of the two types of particles in a single stage regenerator. Though such a process is simple, there are several practical disadvantages, which limit its resid-handling capability, namely (I) the regenerator is kept in the dense phase where the average superficial velocity is about 0.7 meter/second. At such a velocity level, the catalyst particles still possess considerable downward gravitational pull. Moreover, there is a sufficient turbulence and mixing in the bed, which leads to poor segregation efficiency (II) it is known in the FCC art that vanadium is highly mobile in the regenerator atmosphere, and that too in the single stage regenerator, the vanadium may escape from the demetallizing additive to the catalyst particle at these conditions. This defeats the basic purpose of eliminating catalyst deactivation due to metal poisoning.
Haddad et al has addressed some of these issues in U.S. Pat. No. 4,875,994 where combustor type two-stage regenerator is proposed. High velocity combustion air is used to lift the catalyst particles from the combustor. However, the mobile vanadium vapors are allowed to move to the high temperature regenerator through lift line along with the catalyst, which may cause considerable damage to zeolite in the catalyst particles. In addition, the downcomer line from the regenerator to the combustor may allow the separated catalyst particle to again get mixed with the additive. When very high CCR feed is processed, 1st stage regenerator is expected to see high dense bed temperature. As the additive cooler is provided at downstream of first stage regenerator, it is difficult to control dense bed temperature, which will further aggravate the destruction of zeolite structure.
U.S. Pat. No. 4,814,068 discloses a multistage process with three sets of intermediate riser, U bend, mixing and flue gas system. Such a scheme is used to separate large pore catalyst particles from those having intermediate pores in regenerator. The purpose is to reduce hydrothermal deactivation of ZSM-5 additive. The particle size of the coarse particles is also very high (500-70000 microns) to avoid the carryover of coarse particles to the second stage regenerator. The attrition resistance will be poor with such coarse particles.
Similarly, U.S. Pat. Nos. 4,990,314, 4,892,643 and 4,787,967 also cover separation of two very different size particles one having 20-150 micron and the other 500-70,000 microns. Here the stripper section is made annular double stage; thereby the difference in settling velocity of the above two-size range of particles is exploited. The focus was to minimize the frequency of ZSM-5 additive regeneration. However, these inventions do not address the issues related to minimization of metal deactivation of catalyst and removal of feed CCR as arises in residue processing in FCC.
A process and apparatus is disclosed in U.S. Pat. No. 4,830,728 for upgrading naphtha in fluid catalytic cracking operation employing multiple risers with a zeolite Y catalyst and ZSM-5 mixture. Separation of ZSM-5 having particle size in the range of 500-70000 microns from zeolite Y catalyst of particle size range 20-150 microns in FCC stripper. A conical perforated plate or sieve provided at an intermediate location in stripper in such a way that larger & denser ZSM-5 is retained above the plate/sieve and the smaller & lighter catalyst/passes through plate/sieve and settles at the bottom of stripper. The separation mechanism adapted here is different from the present invention. Moreover, this patent does not address the issues pertaining to the problems of avoiding CCR and metal deactivation of catalyst while processing residue in FCC units.
The inventions of U.S. Pat. No. 5,059,302 and U.S. Pat. No. 5,196,172 claim of FCC process and apparatus employing a separable mixture of catalyst and sorbent particles. Here the adsorbent particles are smaller in size (30-90 microns) and the catalyst particles are bigger in size (80-150 micron). The process employs selective vortex pocket classifier and horizontal cyclone type burner/combustor to continuously separate the two types of particles and works based on the difference in centrifugal forces. As the segregation and regeneration of sorbent and catalyst is carried out in first stage regenerator, the vanadium may migrate from the sorbent to the catalyst particle and destroy zeolite structure of the catalyst at such high temperature conditions.
U.S. Pat. No. 6,149,875 deals with removal of CCR and metal contaminants in the heavy feed. However, the apparatus employed in this patent is different from that of present invention. In addition, the difference between transport velocity of catalyst and adsorbent is exploited for segregating catalyst from adsorbent. The superficial gas velocity in separator vessel is very high and is operated in different domain (turbulent/fast fluidization) vis-à-vis bubbling bed regime of the present invention.